1. Technical Field
The present disclosure relates to a microelectromechanical gyroscope with inversion of actuation forces and to a method for actuating a microelectromechanical gyroscope.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has become progressively widespread in various sectors of technology and has yielded encouraging results especially for providing inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures comprising at least one movable mass connected to a fixed body (stator) by springs and movable with respect to the stator according to pre-determined degrees of freedom. The movable mass is moreover coupled to the fixed body via capacitive structures (capacitors). The movement of the movable mass with respect to the fixed body, for example on account of an external stress, modifies the capacitance of the capacitors; from this it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied. Vice versa, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. In addition, to provide electromechanical oscillators, the frequency response of inertial MEMS structures is exploited, which is typically of the second-order low-pass type.
Many MEMS (in particular, all electromechanical oscillators and gyroscopes) include driving devices that have the task of maintaining the movable mass in oscillation.
A first type of known solution envisages supplying, in open loop, periodic excitation at the resonance frequency of the MEMS structure. The solution is simple, but also far from effective, because the resonance frequency is not known with precision on account of the ineliminable dispersions in the processes of micromachining of semiconductors. In addition, the resonance frequency of each individual device can vary over time, for example, on account of temperature gradients or, more simply, on account of ageing.
Feedback driving circuits have then been proposed, based upon the use of sigma-delta modulators. Circuits of this type are undoubtedly more effective than the previous ones in stabilizing the oscillation of the movable mass at the real resonance frequency and in suppressing disturbance.
However, various stages are employed for filtering, decimation, and further processing of the bit flow supplied by the sigma-delta modulator. For this reason, currently available feedback driving circuits are complex to produce, cumbersome and, in practice, costly.
In addition, it should be considered that gyroscopes have a complex electromechanical structure, which comprises two masses that are movable with respect to the stator and are coupled to one another so as to present a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the movable masses is dedicated to driving (driving mass) and is kept in oscillation at the resonance frequency. The other movable mass (sensing mass) is driven in the oscillatory motion and, in the case of rotation of the microstructure with respect to a pre-determined axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the sensing mass operates as an accelerometer that enables sensing of the Coriolis acceleration.
For enabling actuation and providing an electromechanical oscillator in which the sensor performs the role of frequency-selective amplifier, with transfer function of a second-order low-pass type and high merit factor, the driving mass is equipped with two types of differential capacitive structures: driving electrodes and driving-detection electrodes. The driving electrodes have the purpose of sustaining self-oscillation of the movable mass in the direction of actuation, through electrostatic forces generated by the spectral component of the noise at the mechanical resonance frequency of the driving mass. The driving-detection electrodes have the purpose of measuring, through the transduced charge, the position of translation or rotation of the sensing mass in the direction of actuation.
The U.S. Pat. No. 7,305,880 describes a system for controlling the velocity of oscillation of the gyroscope, comprising a differential sense amplifier, a high-pass amplifier, and an actuation and control stage, operating in a continuous-time mode.
The U.S. Pat. No. 7,827,864 describes an improvement of the foregoing control system, in which the control loop comprises a low-pass filter in order to reduce the offset and the effects of parasitic components and couplings by operating on the overall gain and phase of the feedback loop.
In many cases, however, the desire to apply electrostatic forces of a certain intensity for maintaining the driving mass in oscillation prevents reduction of the supply voltages, in contrast with a desire that is increasingly felt in the electronics sector.
In particular, interferences may arise between first voltages, applied to obtain the electrostatic forces necessary to sustain the oscillation, and second voltages, supplied for reading the position of the driving mass.
If the supply voltage is low, the voltage variations of the driving mass can bring about a drop in the voltage on the capacitive coupling between the driving mass and the stator as far as causing inversion of the electrostatic forces. With each period, the driving action tends systematically to dampen instead of sustaining the oscillations of the driving mass, which are insufficient to produce precise readings of the rotations.
In order to make up for the deficiency of electrostatic force, prior art devices have used voltage-boosting circuits (charge pumps) for actuation of the driving mass or else increase the capacitive coupling with a larger number of electrodes. In either case, however, there are disadvantages either from the standpoint of the consumption levels (a fact that can nullify the decrease in the supply voltage) or from the standpoint of area occupation.